Calculations and Links

$total_pages = ceil($total_items / $items_per_page);

//limit results in redirect
$limit_results = "";
if (!empty($section)) {
    $limit_results = "cat=" . $section . "&";
}

// redirect too-large page numbers to the last page
if ($current_page > $total_pages) {
    header("location:catalog.php?" 
        . $limit_results 
        . "pg=".$total_pages);
}
// redirect too-small page numbers to the first page
if ($current_page < 1) {
    header("location:catalog.php?"
        . $limit_results
        . "pg=1");
}

//determine the offset (number of items to skip) for the current page
//for example: on page 3 with 8 item per page, the offset would be 16
$offset = ($current_page - 1) * $items_per_page;